Description:
Most industrial motors require AC (alternating current) voltage whose polarity oscillates 60 times a second (for a frequency of 60 Hertz) between positive and negative.
Single phase AC motors have brushes just like DC motors. Most of them have capacitors to assist starting and to establish the direction of rotation. Changing direction implies changes in the capacitors’ connection. Speed is directly proportional to the applied voltage (just like DC motors). Usage is restricted to low-power uses (home appliances like vacuum cleaners, washing machines, coffee grinders, or power tools like rotary hammers etc.).
For higher power requirements, three-phase AC motors are used. These do not have carbon brushes to wear out, their rotors (called squirrel cages) have no electrical connections, thus making them extremely robust and long-lived. Only the stator windings are connected to the power line. They just keep turning as long as their ball bearings are OK.
Changing direction requires swapping any two of the three phase lines. Speed control is done by modifying the frequency of the AC delivered to the motor; this can be done using complex devices called AC drives.